As products get smaller and smaller, there is increased demand for micro-electrical-mechanical systems (MEMS), micro-optical devices and photonic crystals. With this demand, there is an associated increased interest in micro- and nano-machining. Numerous applications exist for MEMS. As a breakthrough technology, allowing unparalleled synergy between previously unrelated fields such as biology and microelectronics, new MEMS applications are emerging at a rapid pace, expanding beyond those currently identified or known. Additional applications in quantum electric devices, micro-optical devices and photonic crystals are also emerging.
Here are a few applications of current interest:
Quantum Electrical Devices
Interest in ideas such as quantum computing have led to the development of devices requiring increasing smaller dimensions, such as cellular automata and coupled quantum dot technologies. Resonant tunneling devices such as resonant tunneling diodes, which may utilize quantum effects of transmission electrons to increase the efficiency of microwave circuits, require particularly fine features.
Micro-Optics
The application of micro-machining techniques to optics has lead to numerous advances in optical fabrication such as gray scale technology. Gray scale technology allows for the creation of a wide variety of shapes allowing for the best optical performance achievable. Traditional binary optics rely on a “stair step” shaped approximation of the ideal surface shape. Gray scale can actually create that ideal shape. Curves, ramps, torroids, or any other shape is possible. Multi-function optics, microlens arrays, diffusers, beam splitters, and laser diode correctors may all benefit from the use of gray scale technology. These optical devices as well as others, including fine pitch gratings for shorter and shorter wavelength light, benefit from increased precision available using micro-machining. Optical MEMS devices including beam shapers, continuous membrane deformable mirrors, moving mirrors for tunable lasers, and scanning two axis tilt mirrors have also emerged due to progress in micro-machining technology.
Photonic Crystals
Photonic crystals represent an artificial form of optical material that may be used to create optical devices with unique properties. Photonic crystals have many optical properties that are analogous to the electrical properties of semiconductor crystals and, thus, may also allow the development of optical circuitry similar to present electrical semiconductor circuitry. The feature sizes used to form photonic crystals and the precise alignment requirements of these features complicate manufacture of these materials. Improved alignment techniques and reduced minimum feature size capabilities for micro-machining systems may lead to further developments in this area.
Biotechnology
MEMS technology has enabling new discoveries in science and engineering such as: polymerase chain reaction (PCR) microsystems for DNA amplification and identification; micro-machined scanning tunneling microscope (STM) probe tips; biochips for detection of hazardous chemical and biological agents; and microsystems for high-throughput drug screening and selection.
Communications
In addition to advances that may result from the use of resonant tunneling devices, high frequency circuits may benefit considerably from the advent of RF-MEMS technology. Electrical components such as inductors and tunable capacitors made using MEMS technology may perform significantly better than their present integrated circuit counterparts. With the integration of such components, the performance of communication circuits may be improved, while the total circuit area, power consumption and cost may be reduced. In addition, a MEMS mechanical switch, as developed by several research groups, may be a key component with huge potential in various microwave circuits. The demonstrated samples of MEMS mechanical switches have quality factors much higher than anything previously available. Reliability, precise tuning, and packaging of RF-MEMS components are to be critical issues that need to be solved before they receive wider acceptance by the market.
Advances in micro-optics and the introduction of new optical devices using photonic crystals may also benefit communications technology.
Accelerometers
MEMS accelerometers are quickly replacing conventional accelerometers for crash air-bag deployment systems in automobiles. The conventional approach uses several bulky accelerometers made of discrete components mounted in the front of the car with separate electronics near the air-bag. MEMS technology has made it possible to integrate the accelerometer and electronics onto a single silicon chip at a cost of ⅕ to 1/10 of the cost of the conventional approach. These MEMS accelerometers are much smaller, more functional, lighter, and more reliable as well, compared to the conventional macro-scale accelerometer elements.
Micro-Circuitry
Reducing the size of electronic circuits is another area in which MEMS technology may affect many fields. As the density of components and connections increases in these microcircuits, the processing tolerances decrease. One challenge in producing micro-circuitry is preventing shorts between components and nano-wires which are located ever closer together. Yields may be significantly increased by micromachining methods with the capability to repair these defects.
This illustrates one particular challenge in micro-machining, how to modify existing micro- or nano-structures (i.e. where the work piece already has complicated microstructures). Micromachining of submicron features has been a domain predominated by electron-beam, ultraviolet beam, and X-ray lithographic machines, as well as focused ion beam machines. These high-cost techniques usually require stringent environmental conditions, such as high vacuum or clean room condition. All the lithographic methods require a series of complicated procedures, which involve generating multiple masks and using photoresist. If a beam processing technique is used, this process requires the beam to be directed accurately at the desired location with a high degree of precision for proper processing. Only four currently available technologies (laser direct writing, focused ion beam writing, micro electric discharge machine, and photochemical etching) have this potential capability. Other techniques (for example ion beam milling) are only desirable for flat wafer processing. However, direct laser writing has additional advantages, including operation in ambient air and low materials dependence.
The emergence of ultrafast lasers makes submicron-level direct writing possible. In late 1999 and early 2000, the capability of femtosecond laser with a UV wavelength of 387 nm to machine ˜200 nm air holes with pitch size of ˜420 nm in plain Si-on-SiO2 substrate was demonstrated. This demonstration met both the feature size (<200 nm) and pitch size (<420 nm) requirements for a 1D waveguide photonic crystal. The next step was to study drilling small holes on narrow waveguides to make a 1D photonic crystal. Ultrafast lasers have proven to be very versatile tools for micro-, nano-machining. Feature sizes as small as ˜250 nm have now been demonstrated using ultrafast laser beam machining. Still alignment of a laser beam to nanostructures on existing microstructures is a difficult issue.
Micromachining of submicron features has been a domain predominated by electron-beam, ultraviolet beam, and X-ray lithographic machines, as well as focused ion beam machines. These high-cost techniques usually require stringent environmental conditions, such as high vacuum or clean room condition. All the lithographic methods require a series of complicated procedures, which involve generating multiple masks and using photoresist.
Laser machining of surfaces using the near-field radiation of a near-field scanning optical microscope (NSOM), sometimes also known as scanning near-field optical microscope, has been proposed as a means of laser machining submicron features. One potential method for micromachining surfaces in this way is disclosed in Japanese Patent Application 2000-51975, LASER MACHINING APPARATUS AND ITS METHOD AND AN OPTICAL ELEMENT MACHINED BY USING SAME, to H. Owari, et al. Owari, et al. disclose using light from a short-wavelength ultraviolet laser that is transmitted through the probe of an atomic force microscope to laser machine an optical grating.